Early Life May Have Breathed Oxygen Before It Filled the Atmosphere

A new study from MIT suggests that some of Earth's earliest life forms may have been breathing oxygen hundreds of millions of years before oxygen became a permanent fixture in the planet's atmosphere. The research, published in Palaeogeography, Palaeoclimatology, Palaeoecology, provides compelling evidence that aerobic respiration—the process of using oxygen to generate energy—evolved much earlier than previously thought.

Tracing Oxygen's Ancient Origins

The study focuses on a critical enzyme called heme-copper oxygen reductases, which is essential for aerobic respiration in virtually all oxygen-breathing organisms today, from bacteria to humans. By analyzing the genetic sequences of this enzyme across thousands of modern species and mapping them onto an evolutionary tree of life, researchers discovered that the enzyme first appeared during the Mesoarchean era—a geological period spanning from 3.2 to 2.8 billion years ago.

This timing is significant because it predates the Great Oxidation Event (GOE) by several hundred million years. The GOE, which occurred around 2.33 billion years ago, marks when oxygen finally accumulated enough to persist in Earth's atmosphere permanently.

The Cyanobacteria Puzzle

Scientists have long known that cyanobacteria—microbes capable of photosynthesis—emerged around 2.9 billion years ago. These organisms were the first to produce oxygen as a byproduct of converting sunlight and water into energy. Yet oxygen didn't accumulate in the atmosphere until hundreds of millions of years later, creating a puzzling delay.

"We know that the microorganisms that produce oxygen were around well before the Great Oxidation Event," explains Fatima Husain, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences and co-author of the study. "So it was natural to ask, was there any life around at that time that could have been capable of using that oxygen for aerobic respiration?"

Early Oxygen Consumers

The research suggests that as soon as cyanobacteria began producing small amounts of oxygen, other organisms evolved the ability to consume it. These early aerobic organisms likely lived in close proximity to cyanobacteria and quickly gobbled up any oxygen they produced.

This biological consumption of oxygen may have played a crucial role in delaying atmospheric oxygen accumulation. While scientists previously suspected that geochemical reactions with rocks might have drawn down early oxygen, this study adds a biological dimension to the explanation.

Methodological Breakthrough

Tracing the evolutionary history of such an ancient enzyme presented significant challenges. The heme-copper oxygen reductase enzyme is found in the vast majority of modern aerobic organisms, creating an enormous dataset to analyze.

"The hardest part of this work was that we had too much data," says Gregory Fournier, associate professor of geobiology at MIT and another co-author. "This enzyme is just everywhere and is present in most modern living organisms. So we had to sample and filter the data down to a dataset that was representative of the diversity of modern life and also small enough to do computation with, which is not trivial."

The team used molecular clock techniques, combining genetic data with fossil records to estimate when the enzyme first evolved. By placing "pins" on the evolutionary tree based on known fossil ages, they could tighten their estimates for when the enzyme appeared in different lineages.

Implications for Earth's History

This discovery dramatically changes our understanding of aerobic respiration's role in Earth's history. Rather than being a relatively recent evolutionary innovation that emerged after oxygen became abundant, aerobic respiration appears to have co-evolved with oxygen production from the very beginning.

"This does dramatically change the story of aerobic respiration," Husain notes. "Our study adds to this very recently emerging story that life may have used oxygen much earlier than previously thought. It shows us how incredibly innovative life is at all periods in Earth's history."

Filling in the Gaps

The findings help complete a picture of how Earth's atmosphere became oxygenated. Previous MIT research had established the timing of cyanobacteria's emergence and the GOE itself. This new study fills in the critical gap between these events, suggesting that biological oxygen consumption played a significant role in the delay.

"Considered all together, MIT research has filled in the gaps in our knowledge of how Earth's oxygenation proceeded," Husain says. "The puzzle pieces are fitting together and really underscore how life was able to diversify and live in this new, oxygenated world."

Broader Significance

Understanding when and how organisms evolved to use oxygen has implications beyond Earth's history. It provides insights into the conditions necessary for complex life to develop and could inform the search for life on other planets.

The study also highlights the remarkable adaptability of life. Even in an environment with only trace amounts of oxygen, organisms found ways to exploit this resource, demonstrating life's capacity to innovate under challenging conditions.

This research was supported, in part, by the Research Corporation for Science Advancement Scialog program, continuing MIT's long tradition of groundbreaking work in understanding Earth's atmospheric evolution and the origins of life.